1. Field of the Invention
The present invention relates to a method of coating cathode wires by cataphoretically coating wires with an emissive material.
2. Description of the Prior Art
A method of coating cathode wires by cataphoretically coating wires with an emissive material is known from DE-OS No. 27 15 242 which corresponds to U.S. Pat. No. 4,100,449.
In directly heated oxide cathodes, wires are used which are generally made of tungsten and are coated with an emissive material. Since the oxide cathode temperature and, thus, the emittance must be maintained very precisely over the entire length, coated oxide cathode wires are required which have a substantially uniform diameter over their entire length. To that end, tungsten wires are used which are substantially uniform in cross section over their entire length. Such uniform cross section can be achieved by using precision drawing dies to produce the wires.
It is much more complicated to obtain a coating of emissive material of uniform thickness over the entire length and the entire circumference of the wire. The coating is performed by cataphoresis; by the use of a highly complex apparatus, an attempt is made to obtain a coating which is as uniform as possible over the entire length and circumference of the wire.
Thus, as is disclosed in DE-OS No. 27 15 242 mentioned above, the wire to be coated is disposed along the axis of a cylindrical coating vessel and surrounded concentrically by a cylindrical counterelectrode. Furthermore, the coating vessel in this coating process is rotated continuously, thus requiring slip ring contacts for the current supply.
Obviously, however, none of these means is sufficient to achieve a perfectly uniform coating over the entire surface of the wire. Apparently, the coating thickness is influenced not only by the coating apparatus, but also by the texture and the surface properties of the wires. An attempt has been made to overcome this disadvantage by subjecting the wires to a pretreatment, particularly to a complex cleaning operation, prior to the coating process. Even by these means, it has not been possible to completely correct diviations in the coating behavior.